Photoionization cross sections for O-like S IX: A Breit-Pauli R-matrix calculation

In this paper we present photoionization cross sections for the lowest five states of O-like S IX (1s²2s²2p^{4 3}P^0,1,2, ¹D₂, ¹S₀). The relativistic Breit-Pauli R-matrix codes were utilized including all terms of the 2s²2p³, 2s2p⁴, 2p⁵, 2s²2p²3s, 3p, 3d and 2s2p³3s, 3p, 3d configurations in the expansion of the collision wavefunction for S X. It was also found that to achieve convergence of the low-lying energy separations of the target levels, an additional 21 configuration functions needed to be included in the configuration interaction expansion, incorporating two-electron excitations from the 2s and 2p shells to the 3s, 3p and 3d shells. The present work thus constitutes the most sophisticated photoionization evaluation for ground and metastable levels of the S IX ion. Direct comparisons have been made with the only available data found on the OPEN-ADAS database between level resolved contributions of the spectrum. This comparison for the background cross section exhibits excellent agreement at all photon energies for each partial photoionization cross section contribution investigated. Finally, the autoionizing bound states arising from numerous open channels have also been investigated and identified using the QB approach, a procedure for analyzing resonances in atomic and molecular collision theory which exploits the analytic properties of R-matrix theory. Major Rydberg resonance series are also presented and tabulated for the dominant linewidths considered.